CONTROL METHODS FOR NOx

COMPARISON/CONTRAST BETWEEN SULFUR AND NITROGEN OXIDES:

COMPARISONS

1. Both react with oxygen and water in the atmosphere to form acids which cause acid rain.

2. Both lead to the formation of PM10 (Particulate Matter 10 micrometers in diameter) in urban areas.

3. Both are released to the atmosphere in large quantities through combustion processes and are currently regulated by the EPA.

4. Sulfur oxides and nitrogen dioxide are respiratory irritants when present in large quantities

CONTRASTS

1. Sulfur oxides come from sulfur contaminants in fuels. However, most nitrogen oxides are formed by the reaction of atmospheric nitrogen with oxygen in high temperature flames. In essence, sulfur oxides are formed from things that we take from the ground and nitrogen oxides are formed from things that are in the air.

2. The formation of nitrogen oxides in a flame can be controlled by altering temperature, oxygen content, or time. However, the same can't be done for sulfur oxides.

3. The ultimate product of sulfur oxides removed in control equipment is CaSO4. CaSO4 is a low-solubility solid that can be placed in landfills. We don't have as much luck with nitrogen oxides. The final products of nitrogen oxide cleanup are N2 and O2 which will be returned to the atmosphere.

CONTROL TECHNIQUES FOR NOx REDUCTION

There are only two way to reduce NOx emissions:

COMBUSTION MODIFICATION This is the most widely used approach to NOx control. Combustion modification involves mixing part of the combustion air with the fuel and burning as much of the fuel that the air will allow. Then, some of the heat from the flames is transferred to whatever is being heated. Next, the remaining air is added and combustion is finished. This is known as two-stage combustion or reburning

One of the major advantages to this technique is that it is cheap. The disadvantages are that it requires a larger firebox without a higher combustion rate. Also, it is difficult to get complete burning of the fuel in the second stage. Therefore, the amount of unburned fuel and/or carbon monoxide in the exhaust gas increases.

POST-FLAME TREATMENT

Many of these processes require the addition of a reducing agent to the combustion gas stream to take oxygen away from NO. In automobile engines, a platinum-rhodium catalyst is used. The reaction is:

2NO + 2CO + p-r catalyst-----------> N2 + 2CO2

On the other hand, for power plants and other large furnaces, there are many choices of reducing agents. However, the most popular is ammonia. The desired conversion reaction is:

6NO + 4NH3 -------------> 5N2 + 6H2O

However, there is always some oxygen present. This oxygen causes reactions like the following:

4NO + 4NH3 +O2 ---------> 4N2 + 6H2O

If the above reaction occurs, the NO2 is reduced by the following reaction:

2NO2 + 4NH3 +O2 ---------> 3N2 + 6H2O

All of these reactions are expensive to carry out. They can occur either over a zeolite catalyst or in a gas stream in a part of a furnace where the temperature is between 1600 and 1800 degrees Fahrenheit. If the temperature is greater than 1800 degrees, the NO content increases rather than decreases, which exactly what we DON'T want. The dominant reaction is:

NH3+O2 ---------> NO + 3/2H2O
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